Generally, a key step in optical measurement technique is to make the probing beam focused onto the sample. Two methods are currently widely used. One method is to separate the last focusing lens from other components and only to adjust the focusing lens to focus the probing beam onto the sample. For example, as shown in FIG. 1, the focusing is achieved by moving the last focusing lens up and down. The other method is to adjust the whole optical measurement system to focus the probing beam on the sample. For example, as shown in FIG. 2, the focusing is achieved by moving the whole optical system up and down (for example, refer to the U.S. Pat. No. 5,747,813 and U.S. Pat. No. 5,486,701).
With the rapid development of semiconductor industry, it becomes very critical to apply optical technology to accurately measure the Critical Dimension (CD), spatial profile and material characteristics of three-dimensional structures formed by the single-layer film or multilayer film on wafers. While detecting the wafers with normal sizes of 150 mm, 200 mm or 300 mm, its surface may not be flat due to various reasons such as film stress. Therefore, when the whole wafer is detected, auto-focusing for each measurement point is a key technique to achieve high accuracy and rapid measurement and ensure production of semiconductor production line. And it is widely known that focusing the broadband probing beam into a small size spot on the sample surface is highly desired. The small size spot allows to measure the micro-structured patterns and broadband probing beam is helpful for better measurement accuracy. There are some issues in the first focusing method in this case: lens usually has chromatic aberration which results in different wavelengths of light focusing on different locations, increasing the error and thus reducing the measurement accuracy. It is also hard to find the lens materials with good transmission in the whole broadband wavelength range. Those skilled in the art can clearly understand that the entire optical system is adjusted by the second focusing method, and it is very difficult to achieve precise measurement due to the requirements and limits on the weight and speed of the system.
For these reasons, a new method was proposed by those skilled in the art, that is, focusing the broadband probing beam on the sample surface by using the curved mirror (for example, refer to U.S. Pat. No. 5,608,526 and U.S. Pat. No. 7,505,133B1, U.S. Patent Application Publication No. 2007/0247624A1 and Chinese Patent Application Publication No. 101467306A). This method has advantages as below: the mirror does not produce chromatic aberration in whole wavelength range, and has high reflectivity in wide wavelength range.
Although the application of curved mirror does not produce chromatic aberration and thus improve the focusing and measurement accuracy, compared with lens, it is more difficult to align the optical path with curved mirror. The adjustment of focus point and spatial orientation of curved mirror was constrained by incident light, often requiring the simultaneous adjustment of the entire optical system for better adjustment and control of the output optical path and focus point. For example, (1) elliptical mirror: While the spatial location of two focus points is relatively fixed, the adjustable range of optical path and focusing position is very limited by adjusting the individual elliptical mirror after the incident light path was corrected. (2) Toroidal mirror: Although the two corresponding focus points can be achieved in a certain range of incident angles, the spatial relationship of the two focus points changes with the relationship between incident light and toroidal mirror, and the correlations between two focus points are complex and it is very difficult to achieve focusing. Another drawback is that its adjustable range is small and is easy to create image aberrations. (3) Off-axis parabolic mirror: The adjustable range is very limited because the aberrations were resulted as the angle of off-axis parabolic mirror change relative to the direction of incident light. While a wide range of the focusing position can be achieved by moving the off-axis parabolic mirror along the direction of the collimated light beam, the relative position of focus point to the off-axis parabolic mirror centre cannot be changed. This also limits the adjustable range of the focus points. In summary, the use of a single curved mirror itself does not produce chromatic aberration, but it is difficult to adjust and control the direction of the optical path and focusing positions by simple adjustment. Furthermore, the polarization state of beam will be changed after reflected by a single mirror. Here, taking an aluminium mirror as an example, the reflectivities Rs and Rp of S and P polarized light at two incident angles are as shown in FIG. 3a. The above two curves represent the reflectivity Rs of S polarized light. The below two curves represent the reflectivity Rp of P polarized light. The solid line corresponds to an incident angle of 45 degrees, and the dotted line corresponds to an incident angle of 50 degrees. As a result, the reflectivities of S and P polarized light are not equal, and changed with the incident angle. The phase difference between the S and P polarized light reflected is as shown in FIG. 3b. The solid line corresponds to an incident angle of 45 degrees, and the dotted line corresponds to an incident angle of 50 degrees. As a result, the phase difference between the S and P polarized light is different, changed with the incident angle, and associated with the wavelength. In short, because the polarization states S and P with the polarization direction orthogonal to each other have different reflectivity and phase change, after broadband beam being reflected by a mirror, the polarization states of broadband beam varies, resulting in the control of the change of beam polarization difficult (for example, refer to U.S. Pat. No. 6,829,049B1 and U.S. Pat. No. 6,667,805).
The polarization control capability of the spectrometer defines the scope of its applications. Take Optical Critical Dimension (OCD) equipment as an example. Such equipment is widely used in integrated circuit manufacturing lines for process controls. The OCD equipment can measure the Critical Dimension (CD), three-dimensional profile of periodic pattern on sample surface, film thickness and optical constants of multilayer materials by measuring reflectance spectra and phase characteristics of the polarized beam from the sample surface and fitting numerical simulation results. For the spectrometer achieving the critical dimension measurement, the focusing system of spectrometer must be able to control the polarization state of the beam in the process of focusing and optical signal collection in order to measure the sample accurately.
Furthermore, two methods are often used for optical measurement of semiconductor film, i.e., absolute reflectivity measurement method and elliptical polarization measurement method. As described in Chinese Patent Applications No. 201110032744.8, when the absolute reflectivity measurement method is used for measurement, a standard sample is needed to measure, and the measurement result of the standard sample is recorded as a reference value. Then, a sample to be tested is measured, and the measurement result of the sample to be tested is compared to the reference value from the measured standard sample to obtain a relatively true value of the sample to be tested. Due to the light source itself, the spectral intensity thereof may be changed (drifted) during the actual measurement. Theoretically, generally assume that the spectral intensity of the light source is exactly the same when the standard sample and the sample to be tested are measured. But actually, since the sample to be tested and the standard sample cannot be measured at the same time, the changed of the spectral intensity of the light source will influence the measurement result.
For these reasons, it is proposed by those skilled in the art that the reference beam is used for calibrating the light source fluctuation. That is, the light emitted from the light source is divided into two beams. One is used as a probing beam to record the optical information of the sample. The other is used as a reference beam. By measuring the reference beam, the spectral intensity of the light source can be recorded respectively when the reference sample and the sample to be tested are measured, to correct the change of the spectral intensity of the light source during measurement and improve the measurement accuracy.
The measurement equipment is often classified into an optical system with normal-incidence relative to the sample surface and an optical system with oblique incidence relative to the sample surface. The optical system with normal-incidence, due to more compact structure, is often integrated with other process equipment to achieve the integration of production and measurement and real-time monitoring. In the prior art, the normal-incidence spectrometer calibrated by reference beam is achieved by two methods as follows:
(1) As shown in FIG. 4, divergent light emitted from the light source 101 is incident on a beam splitter 103 parallelly after passing through the lens 102. The light transmitted by the beam splitter 103 is used as a probing beam, and the light reflected by the beam splitter 103 is used as a reference beam. The probing beam is focused on the surface of the sample 105 after being converged by the lens 104. The reflected light on the surface of the sample 105 is reflected by the lens 104, thereafter, incident on the beam splitter 103 perpendicularly. The probing beam reflected by the beam splitter 103 is converged by the lens 107 and incident on a probe 108 to obtain the reflection spectrum on the sample surface. The reference beam is incident on a planar mirror 106 perpendicularly, and incident on the beam splitter 103 perpendicularly after being reflected by the planar mirror 106. The reference beam is also converged by the lens 107 after being transmitted by the beam splitter 103, and incident on the probe 108 to obtain the reference spectrum containing light source spectral characteristics (for example, refer to U.S. Pat. No. 7,067,818B2, U.S. Pat. No. 7,189,973B2 and U.S. Pat. No. 7,271,394B2, and U.S. Patent Application Publication No. 2005/0002037A1). In this spectrometer, a controlling aperture can be used for selecting the beam needed to be measured. The method has the advantages as follows: the light source fluctuation can be calibrated, but due to the use of the beam splitter, the spectrometer also has the following problems: (i) light transmission is low. The beam must be transmitted and reflected once by the same beam splitter via the light source during the whole measurement to enter the probe. Assume that the beam splitter has 50% of transmittivity and 50% of reflectivity. The maximum light transmission ratio that the probing beam and the reference beam can reach is only 25%. (ii) If the high quality spot and wide spectral range are achieved simultaneously, it is necessary to deal with chromatic dispersion. The complexity and cost of the system are high. (2) A planar mirror is inserted in the optical path, to make a part of light emitted from the light source is incident on the planar mirror, and the other part pass through the edge of the planar mirror. The beam reflected by the planar mirror is incident on the sample surface perpendicularly as probing beam. The beam passing through the edge of the planar mirror is used as reference beam. The probing beam and the reference beam enter two different spectrometers respectively for simultaneous measurement (for example, refer to U.S. Pat. No. 5,747,813 and U.S. Pat. No. 6,374,967B1). The method has the following advantages: the simultaneous measurement of probing beam and reference beam during measurement corrects the spectrum and intensity changes of the light source accurately; the loss of light intensity during measurement is small and the utilization ratio is high. However, due to the use of two different spectrometers, the photoelectric conversion efficiency is not the same, and the wavelength distribution and resolution are also not the same. Therefore, it is not easy to calibrate the system, and the measurement accuracy will be reduced. On the other hand, the optical path has complex structure in the scheme, and cannot be adjusted easily. The two spectrometers increase the volume of the equipment and increase the cost.
When the spectrometer without polarizer was used to measure the sample with periodic structures, as described in Chinese Patent Applications No. 201010270454.2, the incident beam must be natural light because the rotation angle of incident beam cannot be adjusted relative to the anisotropic angle of samples. In theory, the natural light emitted from light source is required to arrive on the sample surface by either maintaining absolutely polarization or passing through none of polarization-sensitive components. The anisotropic samples cannot be measured if polarization states were presently partly; under this circumstance, the measured values change as the anisotropic samples rotate. Therefore, the spectrometer capable of measuring the anisotropic samples while without polarization control demands the high quality of optical elements and the sophisticated adjustment of the optical path. During measurement, the light reflected by the sample is partially polarized. When the beam is incident on the probe, in theory, the polarization of the incident beam either was maintained completely or no polarization-sensitive component was present in the path. For example, if a polarization-sensitive component was encountered in the path, a depolarizer is required, thus it will reduce the signal to noise ratio. Moreover, the above problem cannot be corrected by numerical methods.